Waste heat from automotive (heat) engines is an essential consequence of the thermodynamics of converting the chemical energy of the fuel into mechanical work. In fact, in a practical engine, the greater portion of the fuel heating value must be removed from the engine proper as a result of which the essential engine exhaust and cooling requirements arise. While this heat is practically unavailable directly for producing further work within the engine itself, it can be made available indirectly by various means for relatively high level uses both within the engine, as by compounding of turbocharging, and external to the engine, as by heating, refrigeration (air conditioning) or other auxiliary power applications. In the first use, the waste heat provides the power to alter engine input conditions or to add recovered power to the output, resulting in increased power and/or fuel economy. The latter case represents a saving in engine power by substituting a waste-heat-powered byproduct function for one which would otherwise take some additional prime engine shaft power and fuel to produce it.
The present invention not only combines improved engine cooling with a system for waste-heat-driven heat pumping but it also permits the two types of waste heat performance augmentation to be combined. In particular, high-rate, isothermal jacket cooling is obtained by nucleate boiling heat transfer to generate a motive fluid for a jet-type thermo-compressor operating in true Rankine power cycle fashion. The power produced therefrom constitutes the actuation of a reverse Rankine cycle (vapor compression) heat pump which can be used to heat or cool various media. A particularly important application is the cooling and heating of the engine intake charge. Another is the cooling and heating of vehicle passenger or cargo environments. Some examples will illustrate the utility of the invention.
Private automobiles represent one of the largest single classes of energy consumers in the United States. A significant portion of fuel consumption of automobiles is due to the growing use of air conditioners. Additionally, air conditioning equipment constitutes a significant fraction of the initial cost of the private automobile. Such equipment typically is provided with a reciprocating piston compressor which is mechanically coupled to the engine and is generally complete in itself as an add-on component. The additional vehicle weight and engine power involved requires more fuel consumption and increases fuel consumption by as much as 15%.
On the other hand, waste heat from automotive engines is available virtually free of cost and is responsive to relatively high energy level uses. Within minutes, even at idle, tail pipe temperatures exceed 500.degree. F. Fully 40% of the heating value of total fuel consumption of a spark ignition engine exits at the tail pipe. Another 35 to 40% leaves by the cooling system. At cruising conditions, exhaust gas temperatures exceeding 1200.degree. F. are produced. Engine material temperatures must be maintained at between 150.degree. and 350.degree. F. by the cooling system to ensure engine integrity.
Accordingly, it would be desirable to provide some means of high rate utilization of the engine waste heat to take economic advantage thereof. The present invention provides a means for effectively and economically controlling engine temperatures while utilizing the otherwise wasted heat generated by an automotive engine to produce refrigeration and heating for passenger comfort control without requiring additional shaft power from the engine. Application of the present invention to passenger automobiles can obtain mileage improvements of as much as 2 to 4 mpg when air conditioning is in use.
Turbocharging internal combustion engines produces spectacular gains in specific power. However, the results of charge compression can be greatly offset by the attendant charge temperature rise from compression. High charge temperatures not only diminish the power recovery of turbocharging but also itensify engine cooling requirements inasmuch as the higher engine inlet temperature will cause a greater portion of the recovered exhaust energy to flow as heat into the engine parts rather than contribute to shaft power. A further consequence is an increase in engine octane requirement if a spark ignition type engine is involved.
Charge cooling, i.e. intercooling, aftercooling or evaporative cooling, is usually employed in high performance turbocharging applications. Evaporative cooling, e.g. the use of excess fuel or water injection as in racing and aircraft applications, can result in excessive fuel consumption and/or power loss. Intercooling, sometimes called aftercooling, may require extensive heat transfer surface and volume in the intake system. This usually involves bulky and expensive heat exchangers and the volume of intake charge hold-up may be increased to the point where engine throttle response is adversely affected. A lower temperature medium and a heat rejection system are also required. If ambient air is the lower temperature medium to which the heat is to be rejected, then significant increases in vehicle drag, frontal area and/or power may be required to achieve the required heat flow for a significant degree of charge cooling.
Without intercooling, the torque gain from charge compression has diminishing returns such that at 60 in. Hg. boost, only about half the potential is realizable. The greater the pressure boost, the more effective is the intercooling.
In a marine engine application these difficulties are easily avoided because of the ready availablity of ambient water for heat rejection. Such a medium has the desired properties of low temperature (usually lower than ambient air), high heat capacity, high heat transport rate and facile pumping characteristics.
However, in other transport applications where only an ambient air heat sink is available, substantial difficulties arise in accomplishing a significant amount of charge cooling by the customary method. Engine jacket cooling water has been used as an intermediate heat transfer medium in intercoolers but the effectiveness is limited due to the engine cooling load which must be shared in the heat rejection system (radiator) and also because of the sensible temperature changes which accompany heat transfer. A separate sensible coolant loop whether of the "direct" or "indirect" type is similarly limited. Heat pumping by vapor compression refrigeration overcomes the limitations of both the ambient heat sink and the sensible temperature gradients. However, a significant amount of power is required to drive the heat pump which, if derived from the engine shaft, would greatly offset any thermal advantage.